I'm trying to contact you about this family, as I have a theory about John A's parents. Hoping that you still read the message boards... This John, b Grenville, c 1839 (age/place from mar. details, assumed to be the most accurate of the options available) supposedly had a father John (mar. details) who was born C. E. (1900 census) and presumably before 1819 if we take the most likely birth estimate I have for the son John).
I suspect, he (the father John) was actually the John born QUE, 1819 to William FAIRBAIRN and Jane WANLESS.
The John that married Sarah/Charity was somewhat flexible with his age as I have him variously born c 1835 Canada West, 1839 Grenville, Apr 1842 Canada E, which is quite a spread.
This John, s/o Wm and Jane, married Helen/Ellen LINDSAY and died in Dec 1909 in a rest home in Kamloops, BC.My info doesn’t yet include a son John for John/Helen however, and an 1839 birth would predate both marriage dates I have, (c 1847 and c 1848)
The naming pattern of John and Sarah's children would indicate that John's parents were a John and Helen, assuming that even though 1 generation removed from Scotland they still followed the naming pattern.